AC Mains Power Supplies

AC mains is an almost unlimited energy source in industrial civilizations and is comparably very affordable. This is why most stationary devices are powered by AC mains.

Overview

AC mains has two distinct properties that prevent it from being used directly to power devices in most cases:

1. Alternating Current (AC): This type of current results from how energy is generated. Revolving turbines produce AC rather than DC. Additionally, power plants are typically located far from consumers. Transporting AC power over long distances results in much lower losses compared to DC.

2. High Voltage: Electric power (watts) is the product of voltage (V) and current (A). A higher voltage allows for a lower current to deliver the same power. Typical household voltages (230V in Europe, 110V in the US*) enable smaller household wiring and reduce fire risks since lower current is required. Current, not voltage, causes cables to overheat and melt.

In summary, AC and high voltage are ideal for transporting electrical energy efficiently. However, high voltage also makes this power potentially life-threatening, as it far exceeds the 30V threshold that human skin can safely insulate. Most modern electronic devices require low voltage and DC, anyway.

AC mains power supplies make AC mains power usable. These include power adapters, laptop bricks, USB power supplies, or heavy-duty high-current power supplies. They are also built into most household appliances which plug into AC mains directly.

How They Work

Almost all AC mains power supplies operate using some or all of the following components:

1. A transformer to lower the AC voltage. Since switching power supplies operate at high frequencies (typically above 100 kHz), they allow for much smaller transformers compared to traditional bulky and heavy linear power supplies. Switching power supplies are much more efficient and produce less heat when compared to classic linear power supplies.

   

   A significant portion of the energy savings we enjoy today comes from using modern switching power supplies instead of linear ones.

2. A rectifier to convert AC to a raw (unstabilized) DC voltage.

   

3. A switching power supply circuitry to produce a stable DC output voltage.

   

Fitted into one metal-shielded housing, switching power supplies are compact and used inside many devices.

Some heavy-duty appliances may still use basic linear power supplies, focusing on high current and simplicity, where output voltage precision, size, and weight are secondary concerns. These power supplies may use a large transformer (which operates much less efficiently at the lower frequency of 50/60Hz), simple diodes for rectification, and a few discrete components.

Dangerous Power Supplies

Some AC mains power supplies are designed extremely poorly to cut production costs. These supplies often:

• Are unregulated or insufficiently regulated.
• Use underrated components that overheat under heavy load, posing a fire hazard.
• Violate safety standards by having inadequate internal isolation, risking exposure of dangerous live mains voltage on the low-voltage DC side.

Since AC mains power supplies are safety-critical devices (with risks of shock or fire hazards), this is not an area to cut corners.

Externally, most AC mains power supplies look the same, but their internal build quality varies greatly. DiodeGoneWild regularly tears down cheap AC mains power supplies, exposing the significant quality variations and shocking realities (in the most literal sense).

Identifying Dangerous Power Supplies

When purchasing cheap power supplies from untrusted sources, you should perform the following simple house-keeping checks immediately to identify potential hazards and return the device in time and claim a refund if necessary:

1. Check Weight:
   While modern switching power supplies are smaller and lighter, quality components still contribute to noticeable weight. Bad power supplies are often substantially lighter than high-quality builds. Compare the weight of the suspect device with a trusted power supply, like a laptop charger or a USB adapter from a reputable manufacturer. If the device feels significantly lighter, return it immediately and avoid using it.

2. Test-Drive with Maximum Load:
   Many cheap power supplies are heavily overrated and cannot reliably provide their claimed output.

   • Connect a load close to the power supply’s maximum rated output. Ideally, use an electronic load for precise testing.
   • Monitor the output voltage and device temperature. The output voltage should remain stable within the rated range. For example:
     • A USB power supply should not drop below 4.8V.
     • A 12V power supply should not drop below 11.5V.
   • If the device gets excessively hot (above 70–80°C), return it.

Other Considerations

Beyond safety and performance issues, there are additional reasons to avoid low-quality power supplies:

1. Electromagnetic Interference (EMI):
   Cheap power supplies often lack proper filtering to reduce EMI, leading to interference with other devices.

2. Power Factor Correction (PFC):
   Quality power supplies include PFC to improve efficiency and minimize harmonics. Poorly designed supplies may consume power inefficiently and introduce noise into the grid.

High Voltage Kills

Always remember: Voltages above 30V can kill.. Even if you are experienced and cautious, consider these scenarios:

• A year later, when you rediscover your DIY device in a pile of junk, will you remember the spots “where you shouldn’t touch or pull”?
• Will others (especially children or their friends) know not to poke their fingers into live components they found in the basement?

Key Rule:

Never include AC mains power directly in your projects. Always use a certified external AC mains power supply and limit your device to safe low DC voltages below 30V.

Do it for your own safety. Do it for your liability.

For Professionals:

If you are a certified electrical engineer and must use AC voltages in your designs:

• Ensure proper grounding for all metal housings, connected to the AC mains ground.
• This ensures that if live components are exposed, fault currents flow through the grounded parts, tripping protective devices (like circuit breakers, residual current circuit breaker (RCCB) or earth leakage circuit breaker (ELCB)) and avoiding harm to people.

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